In low voltage integrated circuit (IC) design, information represented by a small differential signal is often superimposed upon a much larger voltage. It is often difficult to extract and amplify the differential signal and cancel the high voltage component. This is a typical situation in DC-to-DC (switching or linear) voltage regulators, or battery chargers. This function is also widely used in multi-phase voltage regulation modules (VRM), integrated circuits (ICs) in which currents in different phases of the circuit must be matched. In such circuits, current on the high side of input power supply has to be measured accurately, while at the same time, the low voltage circuitry must be protected from over-voltages.
A classical approach to measure currents is to use a sensing resistor, often called a shunt resistor, typically on the order of milliohms, so that a millivolt signal can be detected by means of current flow on the order of an ampere.
One arrangement (FIG. 1) connects the sensing resistor Rsense, 104, in series with the high voltage source 102. But this method is not easily adapted to integrated circuits, since the integrated circuit must interface with Rsense at a high voltage level.
Another less costly approach uses a π (pi) resistive network to measure the voltage drop across a shunt resistor. The π resistive network reduces the measured low voltage signal across the shunt resistor by a factor equal to the ratio of the resistors in the π network, thereby reducing the overall accuracy of the measurement. This reduction ratio can be significant because the low voltage signal is only on the order of millivolts.
Another sensing arrangement (FIG. 2) locates the sensing resistor Rsense near the “ground” supply. The circuit 200 comprises a high (Vhv) line 204, a current source I1, 206, a sensing resistor Rsense, 208, connected to an electrical ground and across which the current I1, flows. Vsense is measured across the sensing resistor 208. In this case, the sensing resistor 208 cannot match to the output impedance of a regulator under test (not shown).
An object of the invention is to provide a high voltage to low voltage interface circuit that can measure differential signals superimposed on this high voltage without reducing overall accuracy.